FileInputStream inputStream = new FileInputStream(new File(excelFilePath));
    	      
 	        workbook = new XSSFWorkbook(inputStream);
                Sheet LastSheet = workbook.getSheet("form1");
                 styleFloat.setDataFormat(formatFloat.getFormat("0.0##"));
    	         iterator = LastSheet.iterator();
                     i=0; 
   	             while (iterator.hasNext())
   	             {
   	               Row nextRow = iterator.next();
   	               Iterator<Cell> cellIterator = nextRow.cellIterator();
                       j=0;  	        
    	               while (cellIterator.hasNext())
   	               {
   	                
                         Cell cell = cellIterator.next();
                         if(<условие модификации ячейки>)
                          {cell.setCellStyle(styleFloat);
    			   cell.setCellValue(val2);
                          } 
                         j++;
                       
                        }
                        i++;
                       }   

This Style does not belong to the supplied Workbook Stlyes Source. Are you trying to assign a style from one workbook to the cell of a differnt workbook?

private XSSFWorkbook workbook = new XSSFWorkbook(); 
    	private XSSFCellStyle styleTitle = workbook.createCellStyle();
    	private XSSFCellStyle styleFloat = workbook.createCellStyle();

cell.setCellStyle(cell.getCellStyle());
	      cell.setCellValue(val2);